Superconductivity The Research and Development of Superconducting Quantum Computers

Jaw-Shen Tsai (Superconducting Quantum Simulation Research Team Team Leader)

The superconducting state is a mysterious phenomenon in which the resistance of metal is zero. In this state, all conduction electrons fall into a special quantum state called a single “macroscopic quantum state.” In the macroscopic quantum state, the degrees of freedom of the conduction electrons are limited to only the macroscopic phase and the charge number, and a well-ordered simple physical system is formed.
Like electrons bound by an atomic potential, the quantization of energy occurs in bound quanta. In a superconducting macroscopic quantum state, an energy level appears that is quantized to a macroscopic quantum state by the binding potential generated by a Josephson junction. Different types of superconducting qubits have been created with superconducting circuits containing a Josephson junction.
The first developed superconducting qubit was a circuit called a charge qubit [Nature 398, 786, 1999]. This qubit is characterized by having a superconducting “island” that stores electron pairs. Various improvements were subsequently made to the circuit, and a long-lifetime qubit called a transmon was created. Extensive research has been conducted globally on superconducting quantum computers based on gate operation using this qubit as the base element. The world’s first superconducting quantum computer for commercial use (IBM) and the demonstration of quantum supremacy (Google) were major milestones in global research and development of quantum computing.

In 2003, a “flux qubit” was developed that has a superconducting loop which stores flux quanta. Currently, this quantum bit is applied to quantum annealing circuits. 4000 bit-class quantum annealing integrated circuits have already been developed by D-Wave Systems and are commercially available.
In our laboratory we conduct research and development of various aspects of quantum computers. To resolve the issue of three-dimensional wiring that occurs in integrating qubits, we have proposed a new scalable micro-architecture for quantum computers using a quasi-two-dimensional network [New Journal of Physics, 22, 043013, 2020] and have prototyped an integrated quantum chip based on this architecture. Characteristically, this circuit technique allows all qubits to be placed on the periphery of the chip with nearest-neighbor coupling between the bits maintained. This qubit placement thus has the advantage of implementing I/O wiring using the conventional two-dimensional broadband wiring technique.
The Figure shows an integrated superconducting qubit computer chip (4 x 4 array equivalent) that uses our prototyped 16-bit chip quasi-two-dimensional network. We are evaluating the performance of the circuit and conducting research aiming at further integration.
We are also conducting research on superconducting quantum circuits while using a cluster state. In this research, we succeeded for the first time in creating a one-dimensional cluster state in the time domain along a superconducting path [arXiv:2105.08609]. By reusing the qubits repeatedly in the time domain, we successfully created a 4-qubit equivalent linear cluster state from only two physical qubits with fidelity as high as 59%. We also examined the expectation value for projector witness and confirmed that the created four qubits are in a genuine multipartite entanglement (GME) state. This shows that an entangled state of qubits that exceeds the number of physical superconducting qubits can be created. The complexity of the qubit’s spatial domain is thus reduced, making it possible to create, in the future, a three-dimensional cluster that allows for error correction. We have prototyped a quantum chip that creates an 18-bit cluster state using a quasi-two-dimensional network.
We are also conducting research on quantum annealing circuits using superconducting qubits. We have proposed a new architecture for fully coupling quantum annealing circuits using a resonator network [J. Phys. Soc. Jap. 88, 061011, 2019]. We prototyped an 8-bit full-coupling quantum annealing chip based on this architecture and are evaluating its performance. Figure: 16-bit integrated superconducting qubit chip using a quasi-two-dimensional network: the circuit is equivalent to a 4 x 4 array; the figure on the left is an enlarged view of the intersection using an airbridge between the qubit and the resonator.